In a transferring or recording process for image data, data is generated by thinning out a part of signals in units of one pixel in order to reduce the amount of data.
For example, in the case of using YCbCr signals made up of a luminance signal Y and chrominance signals Cb and Cr, the amount of data is reduced by data thinning, for example, by adapting the chrominance signals Cb and Cr to single signals in units of several pixels.
A YCbCr image is image data corresponding to a color space expressed using the luminance signal Y and the two chrominance signals (Cb and Cr).
Cb represents a signal obtained by multiplying (B−Y) obtained by subtracting luminance Y from a B signal by a constant and Cr represents a signal obtained by multiplying (R−Y) obtained by subtracting luminance Y from an R signal by a constant.
Note that, although YCbCr is sometimes written as YPbPr, both YCbCR and YPbPr mean substantially the same color signal. In addition, YCbCr is also expressed as YUV in some cases.
In the following explanation, the description will be given of processes for the YCbCr signals. However, the processes of the present application are processes also applicable to each of the YUV and YPbPr signals.
At the time of transmission and recording of the YCbCr signals, a signal amount reduction process is performed in some cases. Specifically, a method of thinning out chrominance (Cb and Cr) components is used in many cases. As human eyes are more sensitive to changes in brightness than changes in color, a process of reducing the amount of data by thinning out the information amount of the chrominance components CbCr without reducing a luminance component Y is widely performed.
Widely used sampling formats include the following types.
(1) 4:4:4 sampling
This sampling format is of a format that holds respective YCbCr signals of each pixel wholly without thinning out.
(2) 4:2:2 sampling
Among the YCbCr signals of each pixel, a Y signal as the luminance signal is held for all the pixels, but CbCr signals as chrominance signals are held for every other pixel in a horizontal direction.
This is a technique adopted for general business video. At the time of reproduction such as printing or display, the chrominance signals CbCr for one vertical line is used as the chrominance signals CbCr for two vertical lines.
(3) 4:2:0 sampling
Among the YCbCr signals of each pixel, the Y signal as the luminance signal is held for all the pixels, but only single CbCr signals as the chrominance signals are held for every four pixels.
Specifically, only the Cb signal is acquired at an odd-numbered scanning line and the Cr signal is acquired at an even-numbered scanning line in a first frame; conversely, only the Cr signal is acquired at an odd-numbered scanning line and only the Cb signal is acquired at an even-numbered scanning line in a second frame. In this manner, the CbCr signals are acquired by thinning out on a scanning line basis. At the time of reproduction, for example, a chrominance signal at a pixel position where there is no chrominance signal is interpolated using a chrominance signal at an adjacent pixel position on the scanning line.
This is the main streaming technique for home digital video and is used in a format such as moving picture experts group (MPEG) compression format and advanced video coding high definition (AVCHD) used in a digital versatile disc (DVD) and the like.
In this manner, in the transmission and recording of YCbCr, image data to which a process of thinning out the chrominance signals CbCr from the YCbCr signals is applied, that is, subsampled image data, is transmitted or recorded in order to reduce the signal amount in many cases.
As described above, the reason why the chrominance signals CbCr are subsampled instead of the luminance signal Y is that the vision is insensitive to changes in chrominance as compared with luminance.
In such a subsampled image, since the original CbCr signals are not set at about half or more than half of pixels among constituent pixels of the image, it is necessary to perform an interpolation process to set the CbCr signals at pixel positions missing the chrominance signals CbCr signals, in a case where a display process for a display or printing is performed. This interpolation process is performed, for example, as a process of copying CbCr pixel values at neighboring pixel positions to pixel positions missing the CbCr signals. However, when such a simple copy is performed, a folding component of the chrominance signal is produced in a color edge region which is a color boundary where the color changes, and this leads to a difficulty that a striped pattern in which the color is changed stepwise, that is, so-called banding, is observed.
For example, Patent Document 1 (Japanese Patent Application Laid-Open No. 2014-45398) is cited as a prior art that discloses a configuration for decreasing this banding phenomenon.
This Patent Document 1 discloses a configuration that calculates a similarity between a neighboring pixel region of a correction target pixel and a pixel region of an adjacent vertical line pixel or horizontal pixel line and calculates a corrected pixel value by pixel value combination in which a pixel value contribution ratio is adjusted according to the similarity.
However, if such similarity determination or a correction process is performed for all the pixels, difficulties of growing processing cost and a possibility of bringing about delay of image display are raised.